The present invention relates to systems and methods for controlling induction motors.
One of the most common methods for controlling induction motors is known in the art as indirect rotor flux orientation control. Continuous feedback of motor operation information and various motor parameters are required using this method. For example, rotor position feedback, rotor resistance and inductance are required parameters using this method. Sensor wheels and position sensors are typically used to determine rotor position. Proper slip frequency is maintained based upon rotor resistance, rotor inductance, and phase current. The motor torque can be calculated by measuring the motor current for a given condition.
This type of control methodology is simple and crude. One significant problem that arises using this method of control is that rotor resistance and rotor inductance is affected by the temperature and magnetic saturation and thus the motor performance is affected as well. Typically, however, it is assumed that rotor resistance and inductance stays constant for all conditions. This assumption is, of course, incorrect and thus the performance of the motor suffers when the rotor is hot.
While there are systems and methods for providing position sensorless control of induction motors, they are typically complicated and their effectiveness varies as motor operating conditions change. Generally, complicated math filters or observers are used to estimate critical motor parameters, such as, rotor resistance, rotor inductance, rotor electrical frequency, etc. As the result, the inaccuracy of estimations greatly effects the motor""s performance. Thus they do not provide optimal dynamic motor control.
Therefore, there exists a need for a new and improved method and system for controlling an induction motor. The new and improved method and system should not depend on continuous position sensor feedback and various motor parameters, since these parameters vary with temperature, magnetic saturation, and motor wear. Further, the new and improved system should allow the motor to operate continuously in an optimized range, require minimum calibration, and accommodate for high motor parameter variation tolerance.
A method for controlling an induction motor using an equivalent circuit model is provided. The equivalent circuit includes a real resistive component and an imaginary inductive component. The method avoids measuring or estimating individual induction motor parameters, instead, only a few operating parameters, such as phase voltages and phase currents are measured to determine a lump sum of the real and imaginary components of the induction motor impedance. Then, a first control function based on the real component of the induction motor impedance is calculated, a second control function based on the imaginary component of the induction motor impedance is calculated. Then, the induction motor excitation frequency is adjusted until the first control function is approximately equal to the second control function. Finally, the magnitude of the phase voltage is varied to achieve the desired motor/generator performance.
In an aspect of the present invention, determining the real component of the induction motor impedance includes calculating the real component of the induction motor impedance using the equation:
Real(Zin)=(Vdsids+Vqsiqs)/(ids2+iqs2). 
In another aspect of the present invention, determining the imaginary component of the induction motor impedance includes calculating the imaginary component of the induction motor impedance using the equation:
Im(Zin)j=(Vqsidsxe2x88x92Vdsiqs)/(ids2+iqs2). 
In still another aspect of the present invention, when the motor is used to convert electrical power to mechanical power, herein referred to as motoring mode, the first control function is calculated using the equation:
Axe2x80x2=Kmxe2x88x92A. 
In still another aspect of the present invention, when motor is used to convert mechanical power to electrical power, herein referred to as generation mode, the first control function is calculated using the equation:
Axe2x80x2=Kg+A. 
In still another aspect of the present invention, calculating a second control function further includes calculating using the following equation for both motoring and generation modes:
Bxe2x80x2=B/(WeKo). 
In still another aspect of the present invention, adjusting an induction motor operating parameter further includes adjusting an excitation frequency.
In still another aspect of the present invention, at the above determined stator excitation frequency, adjusting the amplitude of the voltage applied to motor results in the desired motor torque as described by the equation:                               T          e                =                              3            ⁢                          P              ⁡                              (                                                      Real                    (                                          Z                      in                                        )                                    -                                      R                    s                                                  )                                      ⁢                          (                              V                2                            )                                                          W              e                        ⁡                          (                                                                    (                                          Real                      (                                              Z                        in                                            )                                        )                                    2                                +                                                      (                                          Im                      ⁡                                              (                                                  Z                          in                                                )                                                              )                                    2                                            )                                                          (        3        )            
These and other aspects and advantages of the present invention will become apparent upon reading the following detailed description of the invention in combination with the accompanying drawings.